In gas turbines, maintaining a desired radial clearance between the tips of the rotating blades of the turbine (sometimes called “buckets”) and the facing interior surfaces of the casing is important to performance of the turbine and endurance of the parts. The radial clearance can vary, for example, during transient operation such as start up or stoppage when rotational speed is changing. Also, temperature differences can have an effect on the clearance, not only during such transient operation as individual components are experiencing temperature change, but also during steady state operation as substantial heat is transferred to the turbine section casing internally by hot gas flowing from the combustor section. Casings are commonly constructed from multiple, somewhat non-uniform, arcuate portions arranged circumferentially around the turbine section and attached together, for example, at flanged edges. Accordingly, the circumferentially non-uniform configuration leads to an uneven thermal response around the casing, and non-roundness and local stress concentration can occur as the temperature of the casing changes.
Various strategies have been used be used to control the tip/casing clearance. For example, in some gas turbines, air impingement cooling is used on the outside of the turbine casing to remove heat from the casing, thereby maintaining a more uniform temperature distribution. In such systems an external blower supplies ambient air to manifolds distributed around the casing. Use of such systems incurs capital and operational costs, and also impacts net turbine efficiency.
Achieving a relatively uniform and suitably high heat transfer coefficient across the large, non-uniform, non-standard casing surfaces can be a challenge using such external air impingement. Accordingly, adjustable mounts have been proposed for fine tuning the distance between the casing outer surface and the opposing manifold plate. U.S. Pat. No. 8,123,406 discloses such an adjustable manifold system.
To achieve high heat transfer rates, some gas turbines use manifold plates facing the casing with many small air outlet holes and short nozzle to surface distances. Use of such relatively small impingement cooling holes correspondingly dictates a relatively high differential pressure drop across the holes, thereby requiring cooling air supplied at a higher pressure. Consequently, a higher pressure blower may be needed adding further capital and operational cost, and further negative impact on gas turbine net efficiency. Also, external blowers of the types above can only provide air to the casing at or near room temperature, whereas heating (rather than cooling) of the casing might be desired during some operation conditions. For example, during start up as the mass of the casing is cool and the buckets begin rotating in the hot combustor flow, the tip clearance may be smaller than desired, or the tips may even undesirably contact the inner casing or a shroud element on the inner casing.
In some systems, gas is extracted from the compressor section to cool portions of the turbine section. U.S. Pat. No. 7,690,885 discloses a gas turbine with such compressor gas extraction. Extracted cooling gas passes through plenums and baffles attached to a shroud support, arranged radially outward of a shroud that surrounds the rotating blades or the turbine, to cool the shroud's outer surface. The gas then follows different paths through the shroud to form a film cooling layer along the shroud's inner surface. However, further improvements in thermal management of turbine casings could still be made.